I'm trying to set up deferred lighting in XNA 4. I need to render to 3 multiple render targets. No matter what I do the render target designated as COLOR2 in my pixel shader , the third render target, never renders correctly. I have also loaded tried and tested deferred lighting tutorials and I get the same results....the third RT never renders correctly.

1. XNA 4 hidef is supposed to support 4 MRT's. Is this a limitation of my graphics card?

2. How do I determine the max number of supported MRT's on my target system?

There are a vast number of reasons why things may not be rendering correctly in a multiple render target set-up, what formats are you using for all of your targets, and can you elaborate on what you mean by never rendering correctly, do you get output? what output where you expecting? etc...

With a bit more information we should be able to help you find a solution.

What does the render target look like if it's "not rendered correctly"? Do you get a empty (clear color) target surface? What surface formats are you using? You could try using a test shader that just draws one solid color to see if that's working.